If both parents are carriers for thalessemia, which is an autosomal recessive disorder, what are the chances for pregnancy resulting in an affected child?

Options

A. 0.25
B. 1
C. no chance
D. 0.5

Answer

To determine the chances of a pregnancy resulting in an affected child when both parents are carriers for thalassemia (an autosomal recessive disorder), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Genetic Basis**:
- Thalassemia is an autosomal recessive disorder, meaning that an individual must inherit two copies of the recessive allele (let's denote it as 'a') to express the disease. If an individual has one dominant allele (A) and one recessive allele (a), they are a carrier but do not show symptoms.

2. **Identify the Genotypes of the Parents**:
- Since both parents are carriers, their genotypes can be represented as Aa (where 'A' is the normal allele and 'a' is the thalassemia allele).

3. **Determine Possible Gametes**:
- Each parent can produce two types of gametes: A (dominant) and a (recessive).

4. **Set Up a Punnett Square**:
- Create a Punnett square to visualize the genetic combinations that can occur when these two parents reproduce.
- The Punnett square will look like this:

```
A a
----------------
A | AA | Aa |
----------------
a | Aa | aa |
----------------
```

5. **Analyze the Results**:
- From the Punnett square, we can see the possible genotypes of the offspring:
- AA (normal) - 1 out of 4 (25%)
- Aa (carrier) - 2 out of 4 (50%)
- aa (affected) - 1 out of 4 (25%)

6. **Calculate the Probability of an Affected Child**:
- The probability of having an affected child (aa) is 1 out of 4, or 25%.

### Final Answer:
The chances of a pregnancy resulting in an affected child (thalassemia) when both parents are carriers is **0.25** or **25%**.

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